Source : Webster's Revised Unabridged Dictionary (1913)

Dethronement \De*throne"ment\, n. [Cf. F. d['e]tr[^o]nement.]
   Deposal from a throne; deposition from regal power.

Source : WordNet®

dethronement
     n : the act of deposing someone; removing a powerful person from
         a position or office [syn: {deposition}]